The invention relates to a heater suitable for heating a liquid such as water, or oil.
A heater in which a positive temperature coefficient thermistor (hereinafter, often referred to as "PTC thermistor") is used as a heater element has been well known in the art. Since a PTC thermistor operates at a constant temperature because of its self temperature control function, a heater which is safer and has a high reliability can be realized by using such a PTC thermistor. A heater of this kind has a structure in which a PTC thermistor is sandwiched between a pair of electrode plates and driven through the electrode plates. The electrode plates are thermally coupled indirectly or directly with radiating plates through which heat is taken out. When such a heater is used for heating a liquid such as water or oil, the radiating plates are configured as a part of a case in which basic elements such as the PTC thermistor, and the electrode plates are housed. When a large amount of heat is to be generated, a plurality of PTC thermistors are used, the PTC thermistors are sandwiched between a pair of common electrode plates, and the plural PTC thermistors are simultaneously driven through the electrode plates. Such an assembly structure is disclosed in, for example, Postexamined Japanese Utility Model Publication (Kokuku) Hei-4-36071.
As described above, in such a heater, the electrode plates are used as elements for supplying a power to the PTC thermistor and also as heat conductors. Therefore, it is very important to establish a close electrical connection and thermal coupling between the electrode plates and the PTC thermistors. However, these conditions must surely be satisfied while facilitating assembly and maintaining a simplified structure. This involves technical difficulties. As disclosed in Japanese Utility Model Publication (Kokuku) Hei-4-36071, for example, an electrical connection and thermal coupling between electrode plates and PTC thermistors are improved by using a clamping force exerted by a case. This structure has drawbacks that the structure of the case and the works of assembling the case are complicated. Furthermore, an excessive clamping force may cause the PTC thermistors, which are made of ceramics, to be damaged or crack.
As means for ensuring an electrical connection, a structure has been proposed in which projections are formed on electrode plates and the projections are contacted with electrodes of a PTC thermistor. In this structure, however, gaps the size of which is equal to the height of the projections are formed between the electrode plates and the PTC thermistor, so that the degree of thermal coupling is reduced, thereby lowering the thermal efficiency.
The above-mentioned problems become more noticeable in a structure where a plurality of PTC thermistors are commonly sandwiched between a pair of electrode plates and simultaneously driven.
When a heater for heating a liquid such as water, or oil is to be produced, another technical difficulty arises as follows: In addition to structure a case which functions also as a radiating portion in the liquid, so as to be hermetically sealed to surely block the ingress of the liquid, the electrical contact and thermal coupling between the electrode plates and the PTC thermistor must be improved.